Barely 5ft tall and slightly built, Jing Ping Chen has mastered the art of looking frail and demure. It is a talent that has served her well: few who have met her can believe that she is capable of harsh language, let alone acts of violence.

The truth, however, is rather different. The 37-year-old, better known as Sister Ping, is one of the most ruthless gang leaders in Europe. In the past six years, she has made millions by trafficking human beings from her native China into the UK.

Ping is believed to have smuggled more than 200,000 men and women into the EU and her organisation has been linked to the deaths of 58 Chinese, whose bodies were found in an air-tight truck at Dover in June 2000.

According to the National Criminal Intelligence Service, organised immigration crime is one of the fastest-growing areas of the underworld. At least 600,000 people enter the EU illegally each year and around 80 per cent are brought in by underworld gangs known as 'snakeheads'. Ping was the top snakehead in Europe.

The smuggling industry is as lucrative as the drug trade - across Europe the business is worth £8 billion - but for those who are caught the penalties are far lower.

A spokesman for the National Criminal Intelligence Service told The Observer: 'The top snakeheads control the facilitation process from end to end. They have contacts in China, the UK and at every stop along the route.'

Using a combination of violence and intimidation, Sister Ping swept all her rivals aside and cornered the people-smuggling market between Holland and Britain soon after arriving in Rotterdam in 1997. Through her connections to the Triads - her boyfriend is the head of the Triad gang 14k in Rotterdam - she was able to hire muscle to do her dirty work whenever necessary. In one case, a man had several teeth broken when the barrel of a gun was pushed into his mouth. In another, a man was beaten so badly that he was left with punctured intestines.

When a rival mobster tried to muscle in on Sister Ping's territory, she made a big show of inviting him to the Orient restaurant in Rotterdam's Chinatown, the headquarters of her operation, ostensibly to discuss ways of dividing the territory between them. As soon as he was inside, he was dragged up to the first floor, beaten with a hammer and then shot in both legs.

According to police, Ping's smuggling operation was a well-oiled machine. With at least a dozen people on the payroll, a fleet of eight cars, several other vehicles and at least seven safe houses, the overheads alone were £35,000 per month.

Potential migrants would be approached in China and, after paying a hefty deposit, would be transported via a number of routes to Holland. Once there, they would be held at safe houses for several weeks, waiting for an opportunity to travel to the UK. In the UK they would either claim asylum or be whisked off to work in low-paid, unskilled jobs. Ping's earnings are unknown, but in one recorded telephone conversation a key member of her organisation was heard boasting that he had earned £300,000 in two months. The best estimates suggest that she earned at least £15 million from her criminal activities.

Dutch police picked up Sister Ping's trail after Rotterdam-based Turkish gangster Guersel Ozkam was jailed in Holland in connection with the deaths at Dover. At the time, Ozkam was said to be the head of the smuggling operation and was given a 10-year jail sentence. Although police suspected that he had been taking orders from above, Ozkam refused to name Ping. It was left to Ozkam's ex-girlfriend to contact the police and point them in the right direction.

Within hours of the deaths at Dover, Ping had gone into hiding, but was eventually tracked down through a massive police operation.

Last month a Dutch court sentenced Sister Ping to three years in jail and fined her £8,000 for offences related to human trafficking. Although Judge Pauline Hofmeyer said Ping was 'the leader of a structured group focused on smuggling human beings', Ping was cleared of any personal involvement in the Dover tragedy.

Despite Ping's conviction, the trade in human cargo shows now sign of slowing down. When a small three-bedroom house on the Fairstead Estate in west Norfolk burst into flames in the early hours last month, firemen were shocked to discover 18 Chinese workers sleeping there. All escaped alive.

It has since emerged that at least 1,500 Chinese workers have moved into the King's Lynn area in the past year, most of them working in food processing or farming, often for as little as £2 per hour.

Police intelligence suggests that the snakeheads are now using areas such as Norfolk, Lincolnshire and Cambridgeshire as final destinations for newcomers because there is more work and cheaper housing than can be found in London.

Unscrupulous landlords have been quick to cash in. They charge the migrants up to £40 per person per week. By packing in as many people as possible, properties that would normally rent for little more than £500 a month can generate almost 10 times as much income.

Work is provided by local 'gangmasters', who employ people on a day-to-day basis for cash in hand, regardless of their immigration status. In King's Lynn, dozens of minibuses arrive at the Fairstead Estate each morning to collect hundreds of workers.

The activities of snakehead gangs in the area is also believed to have more sinister undertones. Last month officers of Scotland Yard's Serious Crime Group visited west Norfolk as part of their investigation into the shooting of Chinese-born You Yi He, who was murdered in London's Chinatown. Detectives believe his death may have been linked to a turf war between gangs battling to control the human trafficking trade following the closure of Sister Ping's organisation.